Polyethylene produced from gas phase processes with a degree of branching to improve melt rheology are described in EP-A-495099; EP-A-452920; EP-A-676421 and EP-A-659773. WO 96/08520 (Exxon Chemical Patents Inc) discusses gas phase polymerization using low concentrations of scavenger, in other words, no or only a low amount of scavenger in the form of, for example triethyl aluminum, is used in the course of polymerization.
Polyethylene with improved rheology obtained with monocyclopentadienyl compounds are described in WO-A-93/08221.
U.S. Pat. Nos. 5,336,746; 5,525,689; and 5,639,842 (EP-A-495099) produce polyethylene using hafnium metallocene compounds having multidentate ligands (i.e. they have two cyclopentadienyl ring systems connected by a bridge). The specifically named hafnium compounds are bridged. The described polymerization is performed in a batch system. The polymerization is performed with unsupported catalyst in a solution phase, although mention is made of vapor phase operation. The properties of the resulting polyethylene include a narrow molecular weight distribution and a Melt Flow Rate (MFR expressed in g per 10 minutes at 190 C. under a load of 2.16 kg) of from 8-50. The abbreviation MFR is used to indicate Melt Flow Rate or Melt Flow Ratio depending on the source. Reference must be made to the original source in case of doubt to determine the meaning of MFR in a particular case.
U.S. Pat. No. 5,374,700 (EP-A-452920) does exemplify the use of supported catalyst for making polyethylene. The polymerization is in the gas phase using triisobutyl aluminum as a scavenger. The transition metal component includes zirconocenes. Example 9 and others use ethylene-bridged bis(indenyl)zirconium as the transition metal compound. Example 10 uses an Al/Zr ratio of 112. The scavenger helps to avoid the effect of adventitious poisons attached to the experimental equipment or introduced with the various components. The melt tension is said to be improved.
WO-A-95/07942 uses monocyclopentadienyl compounds in a gas phase on a support for producing polyethylene. The activator is not methyl alumoxane but a non-coordinating bulky anion first described in U.S. Pat. Nos. 5,278,119; 5,407,884; and 5,403,014 (EP-A-277003 and EP-A-277004). Polymerization was performed in a batch reactor. Scavenger was not mentioned.
U.S. Pat. No. 5466649 describes in Example 17 preparing polyethylene using a batch gas phase polymerization procedure using dimethylsilyl bis(tetrahydroindenyl) zirconium dichloride on one support and TMA (trimethyl aluminum) supported separately on another support. This was a batch reaction and no detailed indication of the polyethylene properties was given. U.S. patent application Ser. No. 08/306,055, incorporated by reference, (WO 96/08520) describes a continuous commercial gas phase operating process in which scavenger is either not present or is present in a reduced amount. One embodiment (see page 12, line 28) defines a system essentially free of scavenger, i.e. containing less than 10 ppm of scavenger based on the total weight of the feed gas, which is there referred to as the recycle stream. Alternatively, the low scavenger condition is defined in relation to the metallocene. On page 14, a molar ratio is defined of from 300 to 10. On page 15 it is indicated that the number of olefinic or unsaturated oligomers in the resulting polymer is greatly reduced.
EP-A-676421 exemplifies a batch type process and a continuous process for producing polyethylene which leads to an improved rheology product through introduction of long chain branching by the use of a supported bis-cyclopentadienyl transition metal compound having an alkylene or silyl bridge used in conjunction with a methylalumoxane cocatalyst. The batch reactions are with a scavenger (see page 5, line 28). Example 10 of this patent publication discloses an Melt Index (MI) of 0.3 g per 10 minutes determined at 190 C. under a 2.16 kg load; there is no indication of the molecular weight distribution, the Compositional Distribution is not given, the density is 0.916 g/ml, the Haze is 11%, there is no indication of the ratio of MI's determined under different loads, the Dart Impact Strength is 210 g/mil and there is no indication of the polymer stiffness as expressed by the modulus. On the basis of the correlation between density and secant modulus given in the Encyclopedia of Polymer Science and Engineering, by Mark, Bikales, Overberger, and Menges, Vol. 6, second ed., p.447 (1986), the secant modulus for this material is estimated to be about 30,000 to 32,000 psi (205 to 220 N/mm.sup.2). EP-A-659773 discloses the use of bridged catalysts for the operation of a reactor in a continuous mode for the polymerization of polyethylene. The specification discloses the use of support (see page 6 line 30) but the examples do not use a support so that the alumoxane is in a solvent when injected. The use of an unsupported catalyst may favor fouling and furthermore the alumoxane will contain a significant amount of unreacted trimethyl aluminum (TMA) which may act as a scavenger and lead to an apparent increase in vinyl unsaturation. Melt processing is further influenced by the use of more than one metallocene component which can broaden the molecular weight distribution by the production of more than one distinct polymer component. This is done allegedly to provide control over the degree of long chain branching (LCB) as indicated by the degree of LCB determined by GPC and viscosity data. The melt flow ratio (MFR) is also used to characterize the polymer. The MFR is the ratio of melt index (MI) at different loads and reflects LCB and higher Mw/Mn. Increasing MFR values may be due to higher Mw/Mn caused by the use of more than one metallocene. The examples indicate that the bridged species is most instrumental in raising the level of LCB. However Example 5, which shows the use of the bridged metallocene alone, produces a polymer having a very low molecular weight, suggesting that the low molecular weight polymer species are a major contributor to higher MFR values. EP-A-659773 thus fails to teach how a low melt index material may be produced which has the improved rheology as expressed in MFR resulting from the presence of LCB. EP-A-659773 does not disclose the CDBI, haze and DIS values which help determine the commercial quality of the polymer produced.
EP-A-743327 describes the preparation of an ethylene polymer having a high polydispersity index (which can be represented by Mw/Mn) which requires a lower head pressure in extrusion. The improved rheological properties are expressed in terms of RSI (Relaxation Spectrum Index) which is said to be sensitive to molecular weight distribution, molecular weight and long chain branching. The polymerization process details are scant. EP-A-743327 includes as catalyst similar metallocenes to those listed in EP-A-659773.
EP-A-729978 characterizes an ethylene polymer using flow activation energy. The polymer is made using bridged bis cyclopentadienyl catalyst components, with one cyclopentadienyl ring system being a fluorenyl polynuclear ligand structure. The higher activation energy may be the result of higher levels of long chain branching.
Many different process or catalyst options are introduced in the above processes to achieve the desired effect in the melt processing of the resulting polymers. However it is suggested that these processes all suffer from drawbacks which mitigate against commercial implementation in that the catalyst may have low productivity, be prone to fouling in the longer runs used for large scale reactors and/or produce low molecular weight materials. In addition the prior proposals may lead to an undue sacrifice of physical properties such as loss of clarity, increase in extractability, which is detrimental in food contact applications, or loss of film toughness properties such as dart impact strength (DIS).
It is amongst the aims of the invention to provide a relatively simple process for providing commercially desirable polymer from commercial scale plants which has advantageous melt flow properties and balance of strength and stiffness.
The polymer can be produced in prolonged production runs under conditions not likely to lead to fouling.